Mosquitos in Imperial County test positive for St. Louis Encephalitis virus

East County News Service

September 3, 2025 (Imperial County, CA) – Multiple pools of  mosquitos in both Brawley and Holtville have tested positive for Saint Louis Encephalitis (SLE), the Imperial County Public Health Department announced on August 29.

In rare cases, SLE can cause swelling of the brain or spinal cord, leading to long-term disability or death, though most people infected with the disease have no symptoms or less serious symptoms such as fever, headache, nausea, vomiting and tiredness, according to the U.S. Centers for Disease Control (CDC).

St. Louis encephalitis virus is spread to people by the bite of an infected mosquito. There are no vaccines to prevent or medicines to treat this disease.

“While it’s not unusual to detect mosquito activity during the summer months, the identification of multiple positive pools in different areas is a reminder for all of us to stay alert,” stated Stephen Munday, M.D., Imperial County Health Officer. “We want residents to take precautions, like eliminating standing water and using insect repellent, to help reduce the risk of mosquito-borne illness and protect their families and communities.”

While no mosquitos in San Diego County have tested positive for SLE, our region is just west of Imperial County. San Diego County has had mosquitos test positive for West Nile Virus in several communities, prompting widespread aerial spraying to kill mosquito larvae.

Mosquitoes become infected with St, Louis Encephalitis or with West Nile Virus when they feed on infected birds. Individuals can reduce their risk of mosquito-borne diseases by taking the following precautions:

• Limit time outdoors during dawn and early evening.
• When outdoors, wear loose-fitting, light-colored, long-sleeved shirts and pants when mosquitoes are most active (during dusk and dawn).
• Apply insect repellent that contains DEET, Picaridin, or oil of lemon eucalyptus according to label instructions to prevent mosquito bites.
• Make sure that doors and windows have tight-fitting screens. Repair or replace screens that have tears or holes.

• Eliminate all sources of standing water on your property that can support mosquito breeding by:
  • Draining or eliminating old tires, buckets, plastic covers, toys, or any other containers where mosquitoes can breed.
  • Emptying and changing the water in birdbaths, fountains, wading pools, rain barrels, and potted plant trays at least once a week to destroy potential mosquito habitats.
  • Draining or filling temporary pools of water with dirt.
  • Keeping swimming pool water treated and circulating.
    
     

• Contact Vector Control if there is a significant mosquito problem where you live or work.

If you think you or anyone in your household has symptoms that are causing you concern, contact your healthcare provider.

The United States Environmental Protection Agency (EPA) has a useful search tool that the public can use to find the repellent products most appropriate for them and their families. The tool is available at https://www.epa.gov/insect-repellents/find-repellent-right-you

For additional information related to Saint Louis Encephalitis, please visit the California Department of Public Health Department’s website: https://www.cdph.ca.gov/Programs/CID/DCDC/Pages/SLE.aspx